1. Field of the Invention
This invention relates generally to electronic memory, and more particularly, to a method of accurately measuring current in a flash memory cell.
2. Discussion of the Related Art
Recently, self-testing of flash memory cells on-chip (called Built-In-Self-Test, or BIST) has increased in the interest of reducing cost. Some critical BIST tests require accurate measurement of flash memory cell current. Once such current is known, the cell can be trimmed to establish a desired current level therethrough, so that that cell can be used as a reference cell in reading the states of flash memory cells in an array.
A previous approach has been to measure the flash memory cell current off-chip by sending that current to a tester with a Parametric Measurement Unit (PMU). While this approach provides an accurate reading of the current level, this approach is expensive in terms of hardware and tester time. Furthermore, during the current measurement process, the gate voltage of the transistor of the flash memory cell must be set at a known voltage, the best source of which is directly from the BIST. However, this applied gate voltage is subject to corruption by AC noise from the tester, resulting in error-prone current measurement of the cell.
Moving the current measurement on-chip can significantly save test time and cost. In a previous approach, an on-chip current comparator is used for comparing one-to-one the current to be measured with a known reference current. The most practical method of generating an accurate reference current is to pass current through a resistor external to the chip. However, in this situation, the gate voltage is subject to corruption by noise from power/ground bounce and noise injected from using such an external resistor. Furthermore, on-chip current measurement circuits are subject to imperfections in silicon which lead to device mismatches in the current comparator, which may result in error-prone current measurement.
Indeed, imperfections in silicon causing device mismatches commonly cause input offset error which, in generating reference current, may have that current to have less than desired accuracy.
Therefore, what is needed is an approach which overcomes the above problems by accurately measuring current in a flash memory cell and by providing proper DC offset cancellation.